No hacks are allowed. However legit mods, that don't give you an unfair advantage are allowed. This is a short list of mods that are generally allowed, but keep in mind the rules are always in effect, and that a mod being listed here does not guarantee that it is not possible to misuse it. This list is far from exhaustive, you can use your own judgment as to whether a particular mod violates the rules, or if in doubt ask an admin.
Please note that, as with all wiki pages, anyone with an account can edit this page. If in doubt, ask an admin in the game. Admins will speak to you in purple text.
Texture Packs and Shaders
Texture packs are allowed, but they may not allow you to see through solid blocks like stone, dirt, netherrack etc.
Shaders are allowed. Shaders that increase your gamma setting so that the game is always at full brightness are allowed.
Optifine is allowed.
Minimaps are allowed, but they may not show caves, mobs, other players or entities in general. This means mods such as VoxelMap are NOT allowed.
Bots / Automation Mods
Mods that enable client automation are generally not allowed. There may be exceptions for bots that are 100% automatic.
Improved Chat is allowed.
Automatically logging in after getting disconnected, better known as autoreconnect, is not allowed.
Text-only clients are fine, even if they make you hover/stay still. However in that case you may not use any movement in the client.